Refactor the format skills

[mingxwa]

Changes

• Added unit tests for pro::skills::fmt_format when Unicode support is disabled from fmt library to avoid regression.
• Added <version> to proxy.h to avoid including <format> when the header exists but does not comply to the standard.
• Simplified compiler flag management.
• Use accessor (details::enabled_t) to identify whether std::format is enabled for a specific type with clean std::is_base_of_v, rather than relying on metaprogramming techniques like facade_traits::is_invocable, which is intrusive to proxy implementation.
• Merged std::formatter and fmt::formatter implementation into one traits template.

No functional changes.